The invention relates to the field of printing control in a web-fed printing system. More particularly, the invention relates to the determination of a suitable position for a scanner in a web longitudinal position measurement system such as in a cutoff control system.
In a web-fed printing system, a series of repeating images are printed on a web of material, typically paper. In a typical process, the web is slit into two or more ribbons. The ribbons are directed to a folder where they are aligned one on top of the other and then folded in a direction parallel to the direction of web travel. A cutting mechanism cuts the web in a direction transverse to the direction of web movement. A cutoff control system typically operates to control the longitudinal position of the web such that the cutting mechanism cuts the web at the appropriate time in order to properly separate the repeating images on the web.
Early systems for cutoff control operate with control marks separate from the desired printed image. These control marks, in conjunction with a scanner, are utilized in order to determine the longitudinal position of the image on the web relative to the cutting mechanism. More recent cutoff control systems, such as those described in U.S. Pat. Nos. 4,736,446 and 4,882,764, are also operable in a pattern recognition mode, which mode does not require the use of control marks separate from the desired printed image.
The cutoff control systems described in the above-mentioned patents operate to periodically adjust the positional relationship of the web and the cutting mechanism by controlling the movement of a compensation roller with the use of appropriate control signals. More specifically, as the web travels in a longitudinal direction, an optical scanner is used to produce an output signal corresponding to the light reflected from the image on the web passing underneath. The scanned portion of the image is digitally correlated with a reference image previously stored in memory in order to generate a control signal indicative of the longitudinal offset between the scanned portion and the reference image. The pertinent portion of the image used in the correlation is a strip of the image extending in the longitudinal direction, or essentially the portion of the image passing within the field of view of the scanner.
The control signal is fed to a compensation motor which controls the position of a compensation roller. The compensation roller and a pair of cooperating idler rollers are interposed in the web path upstream of the cutting mechanism. The compensation motor is responsive to the control signal and selectively adjusts the position of the compensation roller such that the effective length of the web path between the printing units and the cutting mechanism is increased or decreased as necessary. In this manner, cutoff at the appropriate location between repeating images on the web is achieved.
At press start-up, it is desirable that the correct cut-off position be achieved as soon as possible in order to minimize the amount of paper waste produced due to bad product. Some current folders have a preset capability, which automatically sets the position of the angle bars and compensators to appropriate positions based on the settings of previous runs of a similar configuration. In this manner, cutoff is effected at approximately the right place, i.e., within approximately a quarter of an inch or so (rather than having cutoff occur directly in the middle of the desired image). By using the known distance from the required cutoff position to a mark sensed by the cutoff control system scanner, proper cutoff control may be rapidly achieved at startup.
When the cutoff control system is run in pattern recognition mode, however, because the longitudinal distance from the proper cutoff position to the start of the reference pattern is generally unknown, proper cutoff control at startup has typically required cutoff adjustment by a pressmen. This results in waste of paper during the startup operation. Once the proper cutoff is achieved, the cutoff control system is intended to operate so as to maintain that cutoff position.
Another cutoff control system is described in U.S. Pat. No. 4,719,575. This patent discloses a method for analyzing a printed image on a web to automatically locate a suitable control mark which is a portion of the image. That method includes the steps of storing a longitudinal profile of indicia located on the web surface by scanning the image, and identifying within the sampled profile local peaks of light intensity. The local peaks indicate portions of the image which can be used as a suitable control mark. In summary, the method disclosed in U.S. Pat. No. 4,719,575 locates a control mark within an alley extending in the longitudinal web direction which is suitable for the cutoff control system as described therein.
As previously described, the cutoff control system disclosed in U.S. Pat. No. 4,882,764 is capable of operating in a pattern recognition mode as well as being capable of operating with the use of separate control marks. When using separate control marks, the control marks are generally printed in the margin around the desired image, and the optical scanner is positioned such that these control marks are within the field of view of the scanner. In the pattern recognition mode, the lateral position of the optical scanner on the web is initially positioned by a pressman.